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Omega Centauri

Omega Centauri is an intimate place for stars. The cluster contains millions of stars packed into a ball a few dozen light-years across. On average, the planets in its core are separated by just one-sixth of a light-year. By comparison, the closest star system to the Sun is more than four light-years away. That means the skies of any planet in the cluster would be ablaze with stars.

A recent study, though, found that it’s unlikely that anyone is there to enjoy the view.

The team was led by Stephen Kane of the University of California-Riverside. It used Hubble Space Telescope to study almost a half-million stars in the center of Omega Centauri, which may be the core of a small galaxy that was consumed by the Milky Way.

The researchers calculated the habitable zone of each star — the distance from the star where conditions are best suited for life. They then calculated the interactions between stars.

They found that, on average, each star passes especially close to a neighbor every million years or so. The pull of the passing stars would make the habitable zones unstable — a planet could be pulled out of the zone. So it would be tough for life to form in the core of Omega Centauri — and just as tough for it to survive.

Omega Centauri is bright enough to see with the unaided eye. But you need to be in the southern third of the U.S. to find it. Right now, it’s quite low in the south about 11 p.m. It looks like a fuzzy star.

Centaurus A

Centaurus A is the fifth-brightest galaxy in the night sky, so it’s an easy target for telescopes and binoculars. But it’s what they can’t see that makes Centaurus A one of the most interesting galaxies around.

Images of the galaxy show a bright blob of stars with wide, dark dust lanes in front of it. That suggests that Centaurus A is the result of a recent merger between two galaxies. New stars are being born in bunches along the dust lanes.

In 1949, radio astronomers found that the galaxy was one of the “loudest” around. The radio waves come from the galaxy’s core and from “jets” that shoot into space above and below the core. They’re almost two million light-years long.

The radio waves are powered by a black hole that’s probably about 55 million times the mass of the Sun. Gas is funneled toward the black hole, perhaps as a result of the merger. As it spirals toward the black hole, it’s heated to millions of degrees. That rips atoms apart, giving them an electric charge. Powerful magnetic fields around the black hole then fling some of the particles out into space, causing them to emit radio waves.

As seen from the United States, Centaurus A stands almost due south as night falls now. It’s best viewed from south of about Dallas or Albuquerque, where it stands a little higher in the sky. Through binoculars, the galaxy looks like a hazy patch of light. It’s just a few degrees above Omega Centauri, a bright star cluster. More about that tomorrow.

Alpha Centauri

Like a bear that spends part of its time in hibernation, the Sun goes through cycles of more and less activity. It’s at its most active every 11 years, when it produces large numbers of magnetic storms — dark sunspots and bright flares. And it’s least active about half way between peaks. It’s in one of those quiet cycles right now.

Other stars go through their own magnetic cycles. Two of the best known are Alpha Centauri A and B. They’re just four-and-a-third light-years from Earth. Only their smaller companion, Proxima Centauri, is closer to us.

Star A is a bit bigger and heavier than the Sun. Star B is a little smaller and lighter than the Sun.

X-ray telescopes in space have been watching the system for years. Over the last decade, in fact, Chandra X-Ray Observatory has looked at the system twice a year. Those observations have helped show that both stars have their own magnetic cycles. Star A’s cycle appears to last about 19 years, while B’s is only about eight years.

Chandra also found that any planets around the stars receive relatively low doses of X-rays — good news for any life at Alpha Centauri.

The system is in the constellation Centaurus, which is wheeling quite low across the southern evening sky. To the eye alone, its stars look like a single pinpoint — the third brightest in the night sky. But the system is quite far south. So from the U.S., it’s visible only from Hawaii and far-southern Texas and Florida.

Moon and Saturn

When Apollo astronauts orbited the Moon, they had some quiet time. When they passed behind the Moon, the Moon’s bulk blocked radio chatter from Earth.

That quietness could be good for astronomy in the decades ahead. Radio telescopes on the lunar farside could study the early days of the universe, when the first stars and galaxies were being born.

Astronomers have studied the possibility of placing telescopes on the Moon for decades. Better technology has made it easier to send telescopes into space, though, reducing the need for lunar telescopes.

But one area where the Moon might still have an advantage is studying long radio waves. Earth’s atmosphere blocks some of these waves. And satellites, cell phones, and other technology creates interference.

Astronomers are interested in these waves because they were emitted by the first stars and galaxies, when the universe was less than a billion years old. The expanding universe has stretched them to much longer wavelengths.

Radio telescopes on the Moon could catch these waves. The telescopes would consist of arrays of long antennas spread across many miles. Shielded from terrestrial interference by the Moon, they could provide new insights into the birth of the stars.

Look for the Moon and a bright companion the next couple of mornings. The planet Saturn will be close to the upper left of the Moon at first light tomorrow, and closer to the right of the Moon on Thursday.

Moon and Planets

The giants of the solar system flank the Moon tonight. Jupiter, the largest planet, rises to the upper right of the Moon late this evening. It looks like a brilliant star. And the second-ranked planet, Saturn, follows them in the wee hours of the morning, well to the lower left of the Moon. It’s not nearly as bright as Jupiter, but there aren’t any other bright stars or planets around it, so it should stand out.

Although both planets are big and heavy, they’re a bit puny compared to some planets found in other star systems. Some of those are many times as massive as Jupiter. Yet none of them is much bigger than Jupiter. It seems that Jupiter is near the “sweet spot” for the sizes of giant planets.

That’s thanks to gravity. A more-massive planet has more stuff, which you’d expect to make it bigger as well. But the extra material also adds to the planet’s gravitational pull, which squeezes it more tightly. That makes it shrink.

And as it shrinks, the planet’s interior gets hotter. At about a dozen times the mass of Jupiter, it gets hot enough that the object is classified as a brown dwarf — an intermediate step between planets and stars. And at about 80 times Jupiter’s mass, the object’s core gets hot enough to ignite nuclear fusion — and shine as a star. Even then, though, it’s not much bigger than Jupiter itself — it’s at the “sweet spot” for big planets, brown dwarfs, and small stars.